1. Field of the Invention
The present invention relates to a piezoelectric element, which is used, for example, as an actuator for changing capacity of ink chamber in order to jet ink filled in the ink chamber from an ink nozzle through an ink passage in an ink-jet printer head, a process for producing the piezoelectric element, and an ink-jet printer head which is constructed using the piezoelectric element.
2. Prior Art
An ink-jet printer head is generally composed of a head base, a diaphragm, and an actuator. A part of the head is enlargedly and schematically shown in FIG. 7. A head base 11 is provided with a large number of ink nozzles for jetting ink, a large number of ink passages separately communicating to the respective ink nozzles, and a large number of ink chambers 12 separately communicating to the respective ink passages. (FIG. 7 shows only one of the ink chambers 12, and illustration of the ink passages and the ink nozzles is omitted.) A diaphragm 13 is mounted so as to cover the whole upper face of the head base 11, and the diaphragm 13 closes upper face openings of all the ink chambers 12 of the head base 11. On the diaphragm 13, piezoelectric elements 15 for giving a vibration driving force to the diaphragm 13 are mounted and formed on positions separately corresponding to the respective ink chambers 12. A power source 19 of an actuator 14 provided with a large number of piezoelectric elements 15 is controlled and voltage is applied to a desired and selected piezoelectric element 15, whereby the piezoelectric element 15 is displaced and the part of the diaphragm 13 is vibrated. As a result, capacity of an ink chamber 12 located at the part corresponding to the vibration of the diaphragm 13 is changed, and ink is pushed out from the ink nozzle through the ink passage.
Each of the piezoelectric elements 15 is constructed by forming a piezoelectric film 17 on a surface of a lower electrode 16, and forming an upper electrode 18 on a surface of the piezoelectric film 17 so as to sandwich the piezoelectric film 17 between the lower electrode 16 and the upper electrode 18. The piezoelectric film 17 is generally made of lead titanate zirconate (Pb(Zr, Ti)O3; PZT) or made of a material mainly composed of the composite oxide. The piezoelectric film 17 having such composition is formed by sputtering method, vacuum deposition method, CVD method, laser ablation method, sol-gel method, thick film method (a method using piezoelectric paste), and so on.
For example, the Japanese Patent Publication (unexamined) No. 92897/1997, the Japanese Patent Publication (unexamined) No. 139594/1998, and the Japanese Patent Publication (unexamined) No. 290035/1998 disclose a process for forming a piezoelectric film of a piezoelectric element used in an ink-jet printer head utilizing sol-gel method by applying sol, which contains a piezoelectric material, over and over onto a lower electrode and repeating a heating process.
The Japanese Patent Publication (unexamined) No. 92897/1997 describes the piezoelectric element in which the lower electrode and the piezoelectric film are tightly fitted by arranging such that at least one of A and B may show a maximum value in concentration at the interface between the piezoelectric film having composition of PbZrO3/PbTiO3/Pb(AxBy)O3 (x+y=1; A:Mg and so on, B:Nb and so on) and the lower electrode. The Japanese Patent Publication (unexamined) No. 139594/1998 describes an art of forming the piezoelectric film from a basic layer made of a ternary system piezoelectric member of lead-magnesium-niobate (PMN), lead zirconate and lead titanate, and a partial layer made of a binary system piezoelectric member of lead zirconate and lead titanate which is crystallized at a lower temperature as compared with the basic layer in order to prevent decrease in piezoelectric strain constant value even when the film is thick. The Japanese Patent Publication (unexamined) No. 290035/1998 describes a piezoelectric element in which a lower layer PZT film is formed on the lower electrode using sol-gel method, and thereafter, the lower layer PZT film is pre-annealed, the lower layer PZT film is crystallized so that titanium and lead may be crystallized faster than zirconium from the lower electrode side and that zirconium may easily segregate on the film surface, an upper layer PZT film is formed on the lower layer PZT film using sol-gel method, the PZT films are annealed, the upper layer PZT film is crystallized so that titanium and lead may be crystallized faster than zirconium from the lower layer PZT film side and that zirconium may easily segregate on the film surface, whereby, concentration of contained zirconium is lower on the lower electrode side and higher on the upper electrode side and concentration of contained titanium is higher on the lower electrode side and lower on the upper electrode side in each of the lower layer PZT film and the upper layer PZT film, and the dielectric constant of the piezoelectric film is lowered keeping the piezoelectric strain constant of the piezoelectric film in order to decrease capacitance and lower calorific value.
As disclosed in the Japanese Patent Publication (unexamined) No. 92897/1997, the Japanese Patent Publication (unexamined) No. 139594/1998, and the Japanese Patent Publication (unexamined) No. 290035/1998 respectively, many attempts have been proposed for improving characteristics and reliability in operation of the piezoelectric element used as an actuator in order to improve performance and durability of the ink-jet printer head. However, in the conventional piezoelectric element, composition of the piezoelectric film is uniform through the entire film. It is certain that the Japanese Patent Publication (unexamined) No. 290035/1998 discloses changing concentration of zirconium and titanium, which are contained in the piezoelectric film formed from PZT, in the thickness direction of the film. But, the piezoelectric element described in the Japanese Patent Publication (unexamined) No. 290035/1998 is merely arranged so that zirconium and titanium contained in the piezoelectric film may be distributed with concentration gradient (difference in concentration) in the thickness direction of the film in order that the dielectric constant of the piezoelectric film may be lowered, capacitance may be decreased, and calorific value may be lowered keeping the piezoelectric strain constant of the piezoelectric film. This known piezoelectric element is not arranged to improve piezoelectric characteristic thereof.
The present invention was made to resolve the above-discussed problems and has an object of providing a piezoelectric element capable of improving piezoelectric characteristics of a piezoelectric film, providing a process for suitably producing such a piezoelectric element, and providing an ink-jet printer head in which the piezoelectric element is used as an actuator.
An invention according to claim 1 provides a piezoelectric element in which an upper electrode and a lower electrode are arranged on two sides of a piezoelectric film formed of a composite oxide (PZT) expressed by a chemical formula, Pb(Zr1xe2x80x94xTix)O3 (0.3xe2x89xa6xxe2x89xa60.9) or formed of a material mainly composed of the composite oxide so as to sandwich the piezoelectric film therebetween, wherein concentration ratio of titanium to zirconium in the piezoelectric film is gradually increased or gradually decreased from the upper electrode side to the lower electrode side.
The foregoing expression, xe2x80x9cconcentration ratio of titanium to zirconium in the piezoelectric film is gradually increased or gradually decreased from the upper electrode side to the lower electrode sidexe2x80x9d includes not only a case in which the concentration ratio changes as indicated by the solid line I or broken line II in FIG. 2 and a case in which the concentration ratio changes stepwise as indicated by the solid line III or broken line IV in FIG. 3, but also a case in which the concentration ratio shows a change like a combination of the stepwise change and the linear change as indicated by the solid line V or broken line VI in FIG. 4. In this manner, the concentration ratio is gradually increased or gradually decreased from the upper electrode side to the lower electrode side, and this is confirmed by conducting, for example, a line analysis using x-ray microanalysis (EPMA). The concept of xe2x80x9cconcentration ratio is gradually increased or gradually decreasedxe2x80x9d in the invention also includes a case in which analysis data include minute unevenness caused by noise in elementary analysis, as long as the concentration ratio is gradually increased or gradually decreased from the upper electrode side to the lower electrode side of the piezoelectric film from a macroscopic viewpoint.
An invention according to claim 2 provides the piezoelectric element according to claim 1, wherein a face of the piezoelectric film, the face being in contact with the upper electrode, is formed of PZT expressed by a chemical formula, Pb(Zr1xe2x80x94xTix)O3 (0.45xe2x89xa6x xe2x89xa60.50) or formed of a material mainly composed of the composite oxide, and concentration ratio of titanium to zirconium is gradually increased or gradually decreased toward the lower electrode side.
An invention according to claim 3 provides the piezoelectric element according to claim 1, wherein a face of the piezoelectric film, the face being in contact with the lower electrode, is formed of PZT expressed by a chemical formula, Pb(Zr1xe2x80x94xTix)O3 (0.45xe2x89xa6x xe2x89xa60.50) or formed of a material mainly composed of the composite oxide, and concentration ratio of titanium to zirconium is gradually increased or gradually decreased toward the upper electrode side.
An invention according to claim 4 provides the piezoelectric element according to any of claims 1 to 3, wherein the piezoelectric film is 1 xcexcm to 25 xcexcm in thickness.
An invention according to claim 5 provides the piezoelectric element according to claim 4, wherein the piezoelectric film is 2 xcexcm to 12 xcexcm in thickness.
An invention according to claim 6 provides the piezoelectric element according to any of claims 1 to 5, wherein the piezoelectric film is made of plural layers of PZT thin films which are different in concentration ratio of titanium to zirconium. The foregoing expression, xe2x80x9cPZT thin films of plural layers which are different in concentration ratio of titanium to zirconiumxe2x80x9d means that PZT thin films, which are different in concentration ratio, are formed into layers. It is also preferable that a PZT thin film having a concentration ratio is formed of plural layers having the same concentration ratio. The fact that the piezoelectric film is formed of the PZT thin films which are different in concentration ratio and are formed into layers, can be verified from stepwise change of the concentration ratio in the thickness direction as shown in FIG. 3 and FIG. 4 in elementary analysis of the piezoelectric film.
However, note that even when the piezoelectric film is formed of plural PZT thin films, the interfaces between the respective layers are not always clear. This is because a mass transfer takes place due to thermal diffusion in a burning process for forming the piezoelectric film. The change in concentration ratio in the piezoelectric film actually becomes something like a combination of stepwise change and linear change as shown in FIG. 4 in some cases.
An invention according to claim 7 provides the piezoelectric element according to claim 6, wherein the piezoelectric film is made of PZT thin films of not less than three layers which are different in concentration ratio of titanium to zirconium.
An invention according to claim 8 provides the piezoelectric element according to claim 7, wherein the piezoelectric film is made of PZT thin films of not less than four layers which are different in concentration ratio of titanium to zirconium.
An invention according to claim 9 provides a process for producing a piezoelectric element comprising the steps of forming a lower electrode on a substrate, forming a piezoelectric film, which is composed of PZT expressed by a chemical formula, Pb(Zr1xe2x80x94xTix)O3 (0.3xe2x89xa6xxe2x89xa60.9) or formed of a material mainly composed of the composite oxide, on the lower electrode, and forming an upper electrode on the piezoelectric film, wherein the piezoelectric film is formed by using plural types of compositions which respectively contain PZT and/or a precursor of PZT and are different in concentration ratio of titanium to zirconium and laminating the compositions on the lower electrode in order of concentration ratio of titanium to zirconium that gradually increases or gradually decreases.
An invention according to claim 10 provides the process for producing a piezoelectric element according to claim 9, wherein not less than three types of compositions which are different in concentration ratio of titanium to zirconium are used.
An invention according to claim 11 provides the process for producing a piezoelectric element according to claim 10, wherein not less than four types of compositions which are different in concentration ratio of titanium to zirconium are used.
An invention according to claim 12 provides the process for producing a piezoelectric element according to any of claims 9 to 11, wherein the compositions are pastes.
An invention according to claim 13 provides the process for producing a piezoelectric element according to any of claims 9 to 11, wherein the compositions are solutions.
An invention according to claim 14 provides the process for producing a piezoelectric element according to claim 13, wherein the solutions are prepared using metallic alkoxide and/or metallic salt as starting material.
An invention according to claim 15 provides a process for producing a piezoelectric element comprising the steps of forming a lower electrode on a substrate, forming a piezoelectric film, which is composed of PZT expressed by a chemical formula, Pb(Zr1xe2x80x94xTix)O3 (0.3xe2x89xa6xxe2x89xa60.9) or formed of a material mainly composed of the composite oxide, on the lower electrode, and forming an upper electrode on the piezoelectric film, wherein the piezoelectric film is formed on the lower electrode by vacuum method such as sputtering method, vacuum deposition method, CVD method, and laser ablation method so that concentration ratio of titanium to zirconium in the piezoelectric film may gradually increase or gradually decrease from the upper electrode side to the lower electrode side.
An invention according to claim 16 provides an ink-jet printer head in which one or not less than two ink nozzles are provided, capacity of an ink chamber communicating to the ink nozzle is changed by an actuator, and ink is jetted through the ink nozzle, wherein the piezoelectric element according to any of claims 1 to or 8 is used as the actuator.
In the piezoelectric element according to claim 1, concentration ratio of titanium to zirconium in the piezoelectric film is gradually increased or gradually decreased from the upper electrode side to the lower electrode side, and this improves piezoelectric characteristic of the piezoelectric film such as diminishing occurrence of stress on the interface between the lower electrode and the piezoelectric film or improving leakage characteristic (conditions in association with the change in leakage current amount when applying a voltage to the piezoelectric film). It is also possible to achieve a larger displacement as compared with a conventional element.
The piezoelectric element according to claim 2 has composition in which the face side of the piezoelectric film, which face is in contact with the upper electrode, has the largest piezoelectric (strain) constant of PZT in the vicinity of a morphotropic phase transition boundary, and the piezoelectric film contracts more on the upper electrode side. On the other hand, the piezoelectric film has a smaller piezoelectric constant on the lower electrode side, and occurrence of stress on the interface between the lower electrode and the piezoelectric film is diminished. It is especially possible to achieve an improvement in displacement.
In the piezoelectric element according to claim 3, the composition on the face side of the piezoelectric film, the face being in contact with the lower electrode, is deviated from the composition in which the piezoelectric constant of PZT becomes the largest in the vicinity of the morphotropic phase transition boundary, and this improves leakage characteristic.
In the piezoelectric element according to claim 4, the piezoelectric film is 1 xcexcm to 25 xcexcm in thickness. As a result, there is no more such disadvantage that effective vibration is not achieved by the element due to excessively thin piezoelectric film and does not have sufficient driving force or that a large driving voltage is required for displacement of the element due to excessively thick piezoelectric film.
In the piezoelectric element according to claim 5, the driving force is appropriate and effective vibration is obtained, and displacement of the element does not require any large driving voltage.
In the piezoelectric element according to claim 6, the piezoelectric film is made of PZT thin films of plural layers. As a result, it is possible to relax stress occurring between the respective layers and prevent deterioration of the element.
In the piezoelectric element according to claim 7, the piezoelectric film is made of PZT thin films of not less than three layers. As a result, it is possible to relax stress occurring between the respective layers and prevent deterioration of the element.
In the piezoelectric element according to claim 8, the piezoelectric film is made of PZT thin films of not less than four layers. As a result, it is possible to relax stress occurring between the respective layers and prevent deterioration of the element more effectively.
In the process for producing the piezoelectric element according to claims 9 to 15, the piezoelectric element according to claim 1 is obtained through a relatively simple process.
The ink-jet printer head according to claim 16 is provided with the piezoelectric element having the foregoing characteristics according to claims 1 to 8 as the actuator. As a result, it is possible to obtain improvement in efficiency as well as in durability.